1. Field of the Invention
This invention relates to a method of forming aqueous solutions containing stannous ion releasing compounds having increased stannous ion stability. The aqueous solutions prepared in accordance with the invention are particularly useful in the formation of aqueous toothpaste, aqueous mouthwash and aqueous gel compositions containing stable stannous compounds exhibiting improved antitartar and antiplaque activity.
2. The Prior Art
There have been many proposals in the prior art to incorporate stannous compounds such as stannous fluoride into oral health care products for the purpose of achieving particular clinical benefits such as caries prevention, plaque control and the reduction of gingivitis. Upon association with water or saliva, these stannous compounds release stannous ions which are active against oral bacteria and provide the desired benefits.
Oral care compositions which contain stannous ion releasing compounds such as stannous fluoride exhibit excellent clinical benefits, particularly in the reduction of gingivitis but have not been widely used in aqueous oral care compositions because of stability problems as the stannous ion is unstable and tends to react with other ingredients of the oral care composition to form insoluble inactive tin compounds, thereby reducing the effective amount of available stannous ion in the composition. Presently available commercial oral care compositions contain stannous fluoride in anhydrous gels because of the instability of stannous ion in aqueous environments. For example U.S. Pat. No. 4,418,057 and 4,533,544 disclose a stannous fluoride oral care composition formulated as a non-aqueous gel mixture including anhydrous glycerin and an hydroxyethyl cellulose gelling agent. Total exclusion of moisture from the gel is required to protect the stannous ion.
Copending patent application Ser. No. 350,309 filed Dec. 6, 1994 discloses an aqueous composition for oral care, containing a stannous compound such as stannous fluoride, and a combination of a water soluble pyrophosphate salt and an organic acid compound which are present in the composition in an amount sufficient to effectively stabilize the stannous ion concentration. The composition containing the stabilized stannous compound is found to exhibit improved antitartar(anticalculus), antiplaque and antigingivitis activity.
During the commercial manufacture of aqueous oral compositions such as toothpastes in accordance with the invention of the above-identified patent application, it is the practice to form a "premix" of the liquid and water soluble portions of the paste to facilitate semi-continuous production in the final product mixing tanks. In the manufacture of the aqueous oral composition, it has been found that the aqueous premix solution which is used does not have adequate stability after its preparation to avoid deterioration over the storage period required preparatory to its use in the commercial production of the oral composition, the stannous ion forming insoluble precipitates within 5-10 minutes of formation of the premix.
The present invention produces a premix solution which is stable as to its stannous ion content over extended periods of time and which may be sub-divided and used in the preparation of oral care compositions in which the stannous ion is effectively stabilized.